a. Field of the Invention
The present invention is concerned with a centrifugal blasting apparatus for blasting abrasive particles such as metallic shot against a workpiece to be cleaned or abraded. More particualrly, it relates to an improved centrifugal blasting apparatus including a rotatable blast wheel and a stationary deflector which are designed to produce a widened blast pattern having a uniform distribution of abrasive particles.
B. Description of the Prior Art
A conventional centrifugal blasting apparatus comprises, as shown in FIG. 9, a base frame 81, a casing 82 fixed to this base frame 81, a chute 83 for supplying abrasive particles, a cylindrical deflector or impeller case 84 attached to said chute 83, and a rotatable blast wheel 85 provided concentrically with said cylindrical deflector 84. The blast wheel 85 is rotated via a drive pulley 88 and a drive shaft 89 and includes a vane supporting disk 90 fixed to the shaft 89 and a plurality of vanes 91 which are substantially rectangular in shape. These vanes 91 are attached on one flat surface of the disk 90 radially thereof and spaced equiangularly from each other. In the central opening of the disk 90 is positioned an impeller 87 which is fixed to the shaft 89 and housed in the cylindrical deflector 84. A feed slot 92 which is either square or rectangular in configuration when viewed from directly thereabove is formed locally through the peripheral wall of the cylindrical deflector 84 so as to face the vane 91. Abrasive particles supplied via the chute 83 into the interior of the cylindrical deflector 84 are hurled by the impeller 87 through the feed slot 92 toward one surface of the respective vanes 91. The particles of abrasive are then propelled by the surfaces of the vanes 91 and blasted against the workpiece 86.
The direction of the blast stream produced by the conventional centrifugal blasting apparatus of the type described is virtually perpendicular to the axis of rotation of the blast wheel so that the blast pattern of abrasive particles cannot become much wider than the width w of the vanes 91. Practically, the angle .alpha. of the blast pattern as shown in FIG. 10 is as small as 10.degree. .
Because of this narrow blast pattern, the centrifugal blasting apparatus of the type descirbed tends to undesirably cause an uneven processing of workpieces. For example, when processing a workpiece having a channel section as shown in FIG. 11 while turning it about its center O on a turn table, the blasting apparatus can abrade the entire outer surfaces and the inner surface GH of the workpiece but cannot give a sufficient processing to the inner surfaces FG and HI. When a turning flat plate as shown in FIG. 12 is processed by the use of the above-mentioned conventional blasting apparatus, there also arises an uneven processing because the angle at which the abrasive particles impinge on the portions G and H closer to the center O is substantially different from the angle of impingement on portions F and I more distant from the center and the distances through which the particles travel before impinging the respective portions is different, and thus the processing forces differ.
To eliminate the aforesaid inconvenience, I, the inventor, have proposed, in Japanese Patent Application publication No. Sho 49-13675 published on Apr. 2, 1974, vanes of a blast wheel each comprising a central face of an isosceles triangular configuration and a pair of triangular sloping faces each extending from the isosceles edge of said central face laterally and backwards with respect to the direction of rotation of the blast wheel. The vane has the general configuration of an isosceles trapezoid when viewed from directly thereabove. By the use of the proposed blast wheel having the vanes constructed as above, the blast pattern has been spread in width, i.e. in the direction parallel to the axis of rotation of the blast wheel. However, it has been found that even the proposed blast wheel cannot avoid the uneven processing because of the fact that those abrasive particles thrown from the central portion of the vane have a noticeably different speed from those thrown from the lateral side portions of the vane. Moreover, due to the particular configuration as stated above, the vanes must be supported by a specially formed saucer-like disk. It will be appreciated that the production of such a saucer-like disk is more troublesome and requires more cost than the production of the flat disk used in said conventional centrifugal blasting apparatus.
Vanes of a blast wheel are sometimes thrown off the wheel when vanes are accidentally disengaged from the disk. In such a event, said conventional blasting apparatus has a tendency to throw the vanes toward a workpiece placed within the narrow stream of the abrasive particles because the direction of the flight of the vanes generally corresponds to that of the abrasive particles. As a result, workpieces are sometimes damaged by those vanes thrown by said conventional blasting apparatus. This presents a serious problem particularly in the case of processing of expensive workpieces such as parts for aircraft. In said proposed blasting apparatus, the vanes are not always thrown toward a workpiece, but there is no knowing in which direction the vanes will be thrown. Accordingly, there is still left the fear that the thrown vanes will damage a workpiece.